Corticotropin-releasing factor receptors differentially regulate stress-induced tau phosphorylation

Chronic Social and Psychological Stress Impact Select Neuropathologies in the PS19 Mouse Model of Tauopathy

OBJECTIVE

Despite advances toward understanding the etiology of Alzheimer's disease (AD), it remains unclear which aspects of this disease are affected by environmental factors. Chronic life stress increases the risk of aging-related diseases including AD. The impact of stress on tauopathies remains understudied. We examined the effects of stress elicited by social (chronic subordination stress [CSS]) or psychological/physical (chronic restraint stress [CRS]) factors on the PS19 mouse model of tauopathy.

METHODS

Male PS19 mice (average age, 6.3 months) were randomized to receive CSS or CRS, or to remain as singly housed controls. Behavioral tests were used to assess anxiety-like behaviors and cognitive functions. Immunofluorescence staining and Western blotting analysis were used to measure levels of astrogliosis, microgliosis, and tau burden. Immunohistochemistry was used to assess glucocorticoid receptor expression.

RESULTS

PS19 mice exhibit neuroinflammation (glial fibrillary acidic protein, t tests: p = .0297; allograft inflammatory factor 1, t tests: p = .006) and tau hyperphosphorylation ( t test, p = .0446) in the hippocampus, reduced anxiety (post hoc, p = .046), and cognitive deficits, when compared with wild-type mice. Surprisingly, CRS reduced hippocampal levels of both total tau and phospho-tau S404 ( t test, p = .0116), and attenuated some aspects of both astrogliosis and microgliosis in PS19 mice ( t tests, p = .068-.0003); however, this was not associated with significant changes in neurodegeneration or cognitive function. Anxiety-like behaviors were increased by CRS (post hoc, p = .046). Conversely, CSS impaired spatial learning in Barnes maze without impacting tau phosphorylation or neurodegeneration and having a minimal impact on gliosis.

CONCLUSIONS

Our results demonstrate that social or psychological stress can differentially impact anxiety-like behavior, select cognitive functions, and some aspects of tau-dependent pathology in PS19 male mice, providing entry points for the development of experimental approaches designed to slow AD progression.

Effects of early tooth loss on chronic stress and progression of neuropathogenesis of Alzheimer's disease in adult Alzheimer's model AppNL-G-F mice

Introduction

Alzheimer's disease (AD), the most common neurodegenerative disease, is characterized by accumulated amyloid-β (Aβ) plaques, aggregated phosphorylated tau protein, gliosis-associated neuroinflammation, synaptic dysfunction, and cognitive impairment. Many cohort studies indicate that tooth loss is a risk factor for AD. The detailed mechanisms underlying the association between AD and tooth loss, however, are not yet fully understood.

Methods

We explored the involvement of early tooth loss in the neuropathogenesis of the adult AppNL-G-F mouse AD model. The maxillary molars were extracted bilaterally in 1-month-old male mice soon after tooth eruption.

Results

Plasma corticosterone levels were increased and spatial learning memory was impaired in these mice at 6 months of age. The cerebral cortex and hippocampus of AD mice with extracted teeth showed an increased accumulation of Aβ plaques and phosphorylated tau proteins, and increased secretion of the proinflammatory cytokines, including interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α), accompanied by an increased number of microglia and astrocytes, and decreased synaptophysin expression. AD mice with extracted teeth also had a shorter lifespan than the control mice.

Discussion

These findings revealed that long-term tooth loss is a chronic stressor, activating the recruitment of microglia and astrocytes; exacerbating neuroinflammation, Aβ deposition, phosphorylated tau accumulation, and synaptic dysfunction; and leading to spatial learning and memory impairments in AD model mice.

Sex Differences in Stress Response: Classical Mechanisms and Beyond

Neuropsychiatric disorders, which are associated with stress hormone dysregulation, occur at different rates in men and women. Moreover, nowadays, preclinical and clinical evidence demonstrates that sex and gender can lead to differences in stress responses that predispose males and females to different expressions of similar pathologies. In this curated review, we focus on what is known about sex differences in classic mechanisms of stress response, such as glucocorticoid hormones and corticotrophin-releasing factor (CRF), which are components of the hypothalamicpituitary- adrenal (HPA) axis. Then, we present sex differences in neurotransmitter levels, such as serotonin, dopamine, glutamate and GABA, as well as indices of neurodegeneration, such as amyloid β and Tau. Gonadal hormone effects, such as estrogens and testosterone, are also discussed throughout the review. We also review in detail preclinical data investigating sex differences caused by recentlyrecognized regulators of stress and disease, such as the immune system, genetic and epigenetic mechanisms, as well neurosteroids. Finally, we discuss how understanding sex differences in stress responses, as well as in pharmacology, can be leveraged into novel, more efficacious therapeutics for all. Based on the supporting evidence, it is obvious that incorporating sex as a biological variable into preclinical research is imperative for the understanding and treatment of stress-related neuropsychiatric disorders, such as depression, anxiety and Alzheimer's disease.

Is Gulf War Illness a prolonged early phase tauopathy?

The work of the Gulf War Illness (GWI) Consortium and that of basic and clinical researchers across the USA have resulted in a better understanding in recent years of the pathological basis of GWI, as well as of the mechanisms underlying the disorder. Among the most concerning symptoms suffered by veterans with GWI are cognitive decrements including those related to memory functioning. These decrements are not severe enough to meet dementia criteria, but there is significant concern that the mild cognitive impairment of these veterans will progress to dementia as they become older. Recent studies on GWI using human brain organoids as well as a rat model suggest that one potential cause of the cognitive problems may be elevated levels of tau in the brain, and this is supported by high levels of tau autoantibodies in the blood of veterans with GWI. There is urgency in finding treatments and preventive strategies for these veterans before they progress to dementia, with added value in doing so because their current status may represent an early phase of tauopathy common to many neurodegenerative diseases.

The conceivable role of prolactin hormone in Parkinson disease: The same goal but with different ways

Parkinson disease (PD) is a progressive neurodegenerative disease (NDD) of the brain. It has been reported that prolactin (PRL) hormone plays a differential effect in PD, may be increasing, reduced or unaffected. PRL level is dysregulated in different neurodegenerative disorders including PD. Preclinical and clinical studies pointed out that PRL may has a neuroprotective against PD neuropathology . Though, the mechanistic role of PRL in PD is not fully elucidated. Therefore, the objective of the present review was to clarify the potential role and mechanistic pathway of PRL in PD neuropathology. The present review highlighted that PRL appears to have a neuroprotective effect against PD neuropathology by inhibiting the expression of pro-inflammatory signaling pathways, antioxidant effects and by inhibiting neuroinflammation. Thus, preclinical and clinical studies are warranted in this regard.

Traumatic brain injury and the pathways to cerebral tau accumulation

Tau is a protein that has received national mainstream recognition for its potential negative impact to the brain. This review succinctly provides information on the structure of tau and its normal physiological functions, including in hibernation and changes throughout the estrus cycle. There are many pathways involved in phosphorylating tau including diabetes, stroke, Alzheimer's disease (AD), brain injury, aging, and drug use. The common mechanisms for these processes are put into context with changes observed in mild and repetitive mild traumatic brain injury (TBI). The phosphorylation of tau is a part of the progression to pathology, but the ability for tau to aggregate and propagate is also addressed. Summarizing both the functional and dysfunctional roles of tau can help advance our understanding of this complex protein, improve our care for individuals with a history of TBI, and lead to development of therapeutic interventions to prevent or reverse tau-mediated neurodegeneration.

Potential Neuroprotective Effects of Dietary Omega-3 Fatty Acids on Stress in Alzheimer's Disease

BACKGROUND

A large number of individual potentially modifiable factors are associated with risk for Alzheimer's disease (AD). However, less is known about the interactions between the individual factors.

METHODS

In order to begin to examine the relationship between a pair of factors, we performed a pilot study, surveying patients with AD and controls for stress exposure and dietary omega-3 fatty acid intake to explore their relationship for risk of AD.

RESULTS

For individuals with the greatest stress exposure, omega-3 fatty acid intake was significantly greater in healthy controls than in AD patients. There was no difference among those with low stress exposure.

CONCLUSIONS

These initial results begin to suggest that omega-3 fatty acids may mitigate AD risk in the setting of greater stress exposure. This will need to be examined with larger populations and other pairs of risk factors to better understand these important relationships. Examining how individual risk factors interact will ultimately be important for learning how to optimally decrease the risk of AD.

Restraint or immobilization: a comparison of methodologies for restricting free movement in rodents and their potential impact on physiology and behavior

Restriction of free movement has historically been used as a model for inducing acute and chronic stress in laboratory animals. This paradigm is one of the most widely employed experimental procedures for basic research studies of stress-related disorders. It is easy to implement, and it rarely involves any physical harm to the animal. Many different restraint methods have been developed with variations in the apparatuses used and the degree of limitation of movement. Unfortunately, very few studies directly compare the differential impact of the distinct protocols. Additionally, restraint and immobilization terms are not differentiated and are sometimes used interchangeably in the literature. This review offers evidence of great physiological differences in the impact of distinct restraint procedures in rats and mice and emphasizes the need for a standardized language on this topic. Moreover, it illustrates the necessity of additional systematic studies that compare the effects of the distinct restraint methodologies, which would help to decide better which procedure should be used depending on the objectives of each particular study.

Chronic Stress, Depression, and Alzheimer's Disease: The Triangle of Oblivion

Chronic stress and high levels of the main stress hormones, and glucocorticoids (GC), are implicated in susceptibility to brain pathologies such as depression and Alzheimer's disease (AD), as they promote neural plasticity damage and glial reactivity, which can lead to dendritic/synaptic loss, reduced neurogenesis, mood deficits, and impaired cognition. Moreover, depression is implicated in the development of AD with chronic stress being a potential link between both disorders via common neurobiological underpinnings. Hereby, we summarize and discuss the clinical and preclinical evidence related to the detrimental effect of chronic stress as a precipitator of AD through the activation of pathological mechanisms leading to the accumulation of amyloid β (Aβ) and Tau protein. Given that the modern lifestyle increasingly exposes individuals to high stress loads, it is clear that understanding the mechanistic link(s) between chronic stress, depression, and AD pathogenesis may facilitate the treatment of AD and other stress-related disorders.

Early Chronic Stress Induced Changes within the Locus Coeruleus in Sporadic Alzheimer's Disease

Chronic exposure to stress throughout the lifespan has been the focus of many studies on Alzheimer's disease (AD) because of the similarities between the biological mechanisms involved in chronic stress and the pathophysiology of AD. In fact, the earliest abnormality associated with the disease is the presence of phosphorylated tau protein in locus coeruleus neurons, a brain structure highly responsive to stress and perceived threat. Here, we introduce allostatic load as a useful concept for understanding many of the complex, interacting neuropathological changes involved in the AD degenerative process. In response to chronic stress, aberrant tau proteins that begin to accumulate within the locus coeruleus decades prior to symptom onset appear to represent a primary pathological event in the AD cascade, triggering a wide range of interacting brain changes involving neuronal excitotoxicity, endocrine alterations, inflammation, oxidative stress, and amyloid plaque exacerbation. While it is acknowledged that stress will not necessarily be the major precipitating factor in all cases, early tau-induced changes within the locus coeruleus-norepinephrine pathway suggests that a therapeutic window might exist for preventative measures aimed at managing stress and restoring balance within the HPA axis.

Research Progress on the Etiology and Pathogenesis of Alzheimer's Disease from the Perspective of Chronic Stress

Due to its extremely complex pathogenesis, no effective drugs to prevent, delay progression, or cure Alzheimer's disease (AD) exist at present. The main pathological features of AD are senile plaques composed of β-amyloid, neurofibrillary tangles formed by hyperphosphorylation of the tau protein, and degeneration or loss of neurons in the brain. Many risk factors associated with the onset of AD, including gene mutations, aging, traumatic brain injury, endocrine and cardiovascular diseases, education level, and obesity. Growing evidence points to chronic stress as one of the major risk factors for AD, as it can promote the onset and development of AD-related pathologies via a mechanism that is not well known. The use of murine stress models, including restraint, social isolation, noise, and unpredictable stress, has contributed to improving our understanding of the relationship between chronic stress and AD. This review summarizes the evidence derived from murine models on the pathological features associated with AD and the related molecular mechanisms induced by chronic stress. These results not only provide a retrospective interpretation for understanding the pathogenesis of AD, but also provide a window of opportunity for more effective preventive and identifying therapeutic strategies for stress-induced AD.

Targeting the cannabinoid system to counteract the deleterious effects of stress in Alzheimer’s disease

Alzheimer’s disease is a progressive neurodegenerative disorder characterized histologically in postmortem human brains by the presence of dense protein accumulations known as amyloid plaques and tau tangles. Plaques and tangles develop over decades of aberrant protein processing, post-translational modification, and misfolding throughout an individual’s lifetime. We present a foundation of evidence from the literature that suggests chronic stress is associated with increased disease severity in Alzheimer’s patient populations. Taken together with preclinical evidence that chronic stress signaling can precipitate cellular distress, we argue that chronic psychological stress renders select circuits more vulnerable to amyloid- and tau- related abnormalities. We discuss the ongoing investigation of systemic and cellular processes that maintain the integrity of protein homeostasis in health and in degenerative conditions such as Alzheimer’s disease that have revealed multiple potential therapeutic avenues. For example, the endogenous cannabinoid system traverses the central and peripheral neural systems while simultaneously exerting anti-inflammatory influence over the immune response in the brain and throughout the body. Moreover, the cannabinoid system converges on several stress-integrative neuronal circuits and critical regions of the hypothalamic-pituitary-adrenal axis, with the capacity to dampen responses to psychological and cellular stress. Targeting the cannabinoid system by influencing endogenous processes or exogenously stimulating cannabinoid receptors with natural or synthetic cannabis compounds has been identified as a promising route for Alzheimer’s Disease intervention. We build on our foundational framework focusing on the significance of chronic psychological and cellular stress on the development of Alzheimer’s neuropathology by integrating literature on cannabinoid function and dysfunction within Alzheimer’s Disease and conclude with remarks on optimal strategies for treatment potential.

Differential effects of chronic stress on anxiety-like behavior and contextual fear conditioning in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative brain disorder that leads to severe cognitive and functional impairments. Many AD patients also exhibit neuropsychiatric symptoms, such as anxiety and depression, prior to the clinical diagnosis of dementia. Chronic stress is associated with numerous adverse health consequences and disease states, and AD patients exhibit altered stress systems. Thus, stress may represent a causal link between neuropsychiatric symptoms and AD. To address this possibility, we examined the effects of chronic stress in the TgF344-AD rat model that co-expresses the mutant human amyloid precursor protein (APPsw) and presenilin 1 (PS1ΔE9) genes. Adult male transgenic (Tg+) and wild-type (WT) rats (6-7.5 months of age), with and without a history of chronic restraint stress, were tested for footshock-induced conditioned fear and for anxiety-like behavior in the elevated plus-maze. We found that non-stressed Tg+ rats showed increased anxiety-like behavior compared to non-stressed WT rats. In contrast, Tg+ and WT rats did not differ in levels of freezing immediately following footshock or during contextual re-exposure. Additionally, stressed Tg+ rats were not significantly different from stressed WT rats on any measures of anxiety or fear. Thus, while stress has been linked as a risk factor for AD-related pathology, it appears from the present findings that two weeks of daily restraint stress did not further enhance anxiety- or fear-like behaviors in TgF344-AD rats.

Role of CRF and the hypothalamic-pituitary-adrenal axis in stroke: revisiting temporal considerations and targeting a new generation of therapeutics

Ischaemic neurovascular stroke represents a leading cause of death in the developed world. Preclinical and human epidemiological evidence implicates the corticotropin-releasing factor (CRF) family of neuropeptides as mediators of acute neurovascular injury pathology. Preclinical investigations of the role of CRF, CRF receptors and CRF-dependent activation of the hypothalamic-pituitary-adrenal (HPA) axis have pointed toward a tissue-specific and temporal relationship between activation of these pathways and physiological outcomes. Based on the literature, the major phases of ischaemic stroke aetiology may be separated into an acute phase in which CRF and anti-inflammatory stress signalling are beneficial and a chronic phase in which these contribute to neural degeneration, toxicity and apoptotic signalling. Significant gaps in knowledge remain regarding the pathway, temporality and systemic impact of CRF signalling and stress biology in neurovascular injury progression. Heterogeneity among experimental designs poses a challenge to defining the apparent reciprocal relationship between neurological injury and stress metabolism. Despite these challenges, it is our opinion that the elucidated temporality may be best matched with an antibody against CRF with a half-life of days to weeks as opposed to minutes to hours as with small-molecule CRF receptor antagonists. This state-of-the-art review will take a multipronged approach to explore the expected potential benefit of a CRF antibody by modulating CRF and corticotropin-releasing factor receptor 1 signalling, glucocorticoids and autonomic nervous system activity. Additionally, this review compares the modulation of CRF and HPA axis activity in neuropsychiatric diseases and their counterpart outcomes post-stroke and assess lessons learned from antibody therapies in neurodegenerative diseases.

Prolactin and Its Altered Action in Alzheimer's Disease and Parkinson's Disease

BACKGROUND

Prolactin (PRL) is one of the most diverse pituitary hormones and is known to modulate normal neuronal function and neurodegenerative conditions. Many studies have described the influence that PRL has on the central nervous system and addressed its contribution to neurodegeneration, but little is known about the mechanisms responsible for the effects of PRL on neurodegenerative disorders, especially on Alzheimer's disease (AD) and Parkinson's disease (PD).

SUMMARY

We review and summarize the existing literature and current understanding of the roles of PRL on various PRL aspects of AD and PD.

KEY MESSAGES

In general, PRL is viewed as a promising molecule for the treatment of AD and PD. Modulation of PRL functions and targeting of immune mechanisms are needed to devise preventive or therapeutic strategies.

Chronic Posttraumatic Stress Disorder and Comorbid Cognitive and Physical Impairments in World Trade Center Responders

Posttraumatic stress disorder (PTSD) has been linked to increased prevalence and incidence of cognitive and physical impairment. When comorbid, these conditions may be associated with poor long-term outcomes. We examined associations between chronic PTSD and symptom domains with cognitive and physical functioning in World Trade Center (WTC) responders nearly 20 years after the September 11, 2001, terrorist attacks. Participants included a cross-sectional sample of 4,815 responders who attended a monitoring program in 2015-2018. Montreal Cognitive Assessment scores less than 23 indicated cognitive impairment (CogI); Short Physical Performance Battery scores 9 or lower on a hand-grip test indicated physical impairment (PhysI). Comorbid cognitive/physical impairment (Cog/PhysI) was defined as having cognitive impairment with at least one objective PhysI indicator. Clinical chart review provided PTSD diagnoses; symptom domains were assessed using the PTSD Checklist. Participants were on average 53.05 years (SD = 8.01); 13.44% had PTSD, 7.8% had CogI, 24.8% had PhysI, and 5.92% had comorbid Cog/PhysI. Multivariable-adjusted multinomial logistic regression demonstrated that Responders with PTSD have more than three times the risk of Cog/PhysI (adjusted RR = 3.29, 95% CI 2.44- 4.44). Domain-specific analyses revealed that emotional numbing symptoms predicted an increased risk of PhysI (adjusted RR = 1.57, 95% CI 1.08-2.28), whereas reexperiencing symptoms were associated with comorbid Cog/PhysI (adjusted RR = 3.96, 95% CI, 2.33-6.74). These results suggest that responders with chronic PTSD may have increased risk of deficits beyond age-expected impairment characterized by the emergence of comorbid Cog/PhysI at midlife.

Lifelong chronic psychosocial stress induces a proteomic signature of Alzheimer's disease in wildtype mice

Late onset, sporadic Alzheimer's disease (AD) accounts for the vast majority of cases. Unlike familial AD, the factors that drive the onset of sporadic AD are poorly understood, although aging and stress play a role. The early onset/severity of neuropathology observed in most genetic mouse models of AD hampers the study of the role of aging and environmental factors; thus alternate strategies are necessary to understand the contributions of these factors to sporadic AD. We demonstrate that mice acquiring a low social status (subordinate) in a lifelong chronic psychosocial stress (CPS) model, accrue widespread proteomic changes in the frontal/temporal cortex during aging. To better understand the significance of these stress-induced changes, we compared the differentially expressed proteins (DEPs) of subordinate mice to those of patients at varying stages of dementia. Sixteen and fifteen DEPs upregulated in subordinate mice were also upregulated in patients with mild cognitive impairment (MCI) and AD, respectively. Six of those upregulated proteins (CPE, ERC2, GRIN2B, SLC6A1, SYN1, WFS1) were shared by subordinate mice and patients with MCI or AD. Finally, comparison with a spatially detailed transcriptomic database revealed that the superior frontal gyrus and hippocampus had the greatest overlap between mice subjected to lifelong CPS and AD patients. Overall, most of the overlapping proteins were functionally associated with enhanced NMDA receptor mediated glutamatergic signaling, an excitotoxicity mechanism known to affect neurodegeneration. These findings support the association between stress and AD progression and provide valuable insight into potential early biomarkers and protein mediators of this relationship.

Anxiety and Alzheimer's disease: Behavioral analysis and neural basis in rodent models of Alzheimer's-related neuropathology

Alzheimer's disease (AD) pathology is commonly associated with cognitive decline but is also composed of neuropsychiatric symptoms including psychological distress and alterations in mood, including anxiety and depression. Emotional dysfunction in AD is frequently modeled using tests of anxiety-like behavior in transgenic rodents. These tests often include the elevated plus-maze, light/dark test and open field test. In this review, we describe prototypical behavioral paradigms used to examine emotional dysfunction in transgenic models of AD, specifically anxiety-like behavior. Next, we summarize the results of studies examining anxiety-like behavior in transgenic rodents, noting that the behavioral outcomes using these paradigms have produced inconsistent results. We suggest that future research will benefit from using a battery of tests to examine emotional behavior in transgenic AD models. We conclude by discussing putative, overlapping neurobiological mechanisms underlying AD-related neuropathology, stress and anxiety-like behavior reported in AD models.

COVID-19 and Alzheimer's disease: how one crisis worsens the other

Alzheimer’s disease (AD) has emerged as a key comorbidity of coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). The morbidity and mortality of COVID-19 are elevated in AD due to multiple pathological changes in AD patients such as the excessive expression of viral receptor angiotensin converting enzyme 2 and pro-inflammatory molecules, various AD complications including diabetes, lifestyle alterations in AD, and drug-drug interactions. Meanwhile, COVID-19 has also been reported to cause various neurologic symptoms including cognitive impairment that may ultimately result in AD, probably through the invasion of SARS-CoV-2 into the central nervous system, COVID-19-induced inflammation, long-term hospitalization and delirium, and post-COVID-19 syndrome. In addition, the COVID-19 crisis also worsens behavioral symptoms in uninfected AD patients and poses new challenges for AD prevention. In this review, we first introduce the symptoms and pathogenesis of COVID-19 and AD. Next, we provide a comprehensive discussion on the aggravating effects of AD on COVID-19 and the underlying mechanisms from molecular to social levels. We also highlight the influence of COVID-19 on cognitive function, and propose possible routes of viral invasion into the brain and potential mechanisms underlying the COVID-19-induced cognitive impairment. Last, we summarize the negative impacts of COVID-19 pandemic on uninfected AD patients and dementia prevention.

Trait anxiety, a personality risk factor associated with Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease in elderly population and the leading cause of dementia worldwide. While senile plaques and neurofibrillary tangles have been proposed as the principal histopathologic hallmarks of AD, the exact etiology of this disease is still far from being clearly understood. AD has been recognized as pathological consequences of complex interactions among genetic, aging, medical, life style and psychosocial factors. Recently, the roles of neuroticism personality traits in AD incidence and progression have come into focus. More specifically, increasing evidence has further shown that the trait anxiety, one major component of neuroticism predicting the individual vulnerability in response to stress, is a risk factor for AD and may correlated with various AD pathologies. In this review, we summarized recent literature on the association of trait anxiety with AD. We also discussed the possible neuroendocrinological and neurochemical mechanisms of this association, which may provide clinical implications for AD diagnosis and therapy.

Sleep Deprivation, a Link Between Post-Traumatic Stress Disorder and Alzheimer's Disease

An estimated 5 million Americans are living with Alzheimer's disease (AD), and there is also a significant impact on caregivers, with an additional 16 million Americans providing unpaid care for individuals with AD and other dementias. These numbers are projected to increase in the coming years. While AD is still without a cure, continued research efforts have led to better understanding of pathology and potential risk factors that could be exploited to slow disease progression. A bidirectional relationship between sleep deprivation and AD has been suggested and is well supported by both human and animal studies. Even brief episodes of inadequate sleep have been shown to cause an increase in amyloidβ and tau proteins, both well-established contributors toAD pathology. Sleep deprivation is also the most common consequence of post-traumatic stress disorder (PTSD). Patients with PTSD frequently present with sleep disturbances and also develop dementia at twice the rate of the general population accounting for a disproportionate representation of AD among U.S. Veterans. The goal of this review is to highlight the relationship triad between sleep deprivation, AD, and PTSD as well as their impact on molecular mechanisms driving AD pathology.

Neuroimmune connections between corticotropin-releasing hormone and mast cells: novel strategies for the treatment of neurodegenerative diseases

Corticotropin-releasing hormone is a critical component of the hypothalamic–pituitary–adrenal axis, which plays a major role in the body’s immune response to stress. Mast cells are both sensors and effectors in the interaction between the nervous and immune systems. As first responders to stress, mast cells can initiate, amplify and prolong neuroimmune responses upon activation. Corticotropin-releasing hormone plays a pivotal role in triggering stress responses and related diseases by acting on its receptors in mast cells. Corticotropin-releasing hormone can stimulate mast cell activation, influence the activation of immune cells by peripheral nerves and modulate neuroimmune interactions. The latest evidence shows that the release of corticotropin-releasing hormone induces the degranulation of mast cells under stress conditions, leading to disruption of the blood-brain barrier, which plays an important role in neurological diseases, such as Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, autism spectrum disorder and amyotrophic lateral sclerosis. Recent studies suggest that stress increases intestinal permeability and disrupts the blood-brain barrier through corticotropin-releasing hormone-mediated activation of mast cells, providing new insight into the complex interplay between the brain and gastrointestinal tract. The neuroimmune target of mast cells is the site at which the corticotropin-releasing hormone directly participates in the inflammatory responses of nerve terminals. In this review, we focus on the neuroimmune connections between corticotropin-releasing hormone and mast cells, with the aim of providing novel potential therapeutic targets for inflammatory, autoimmune and nervous system diseases.

Reproductive status impact on tau phosphorylation induced by chronic stress

Sex and exposure to chronic stress have been identified as risk factors for developing Alzheimer's disease (AD). Although AD has been demonstrated to be more prevalent in females, sex is often overlooked in research studies, likely due to the complexity of the hormonal status. In female rats, the reproductive status can modulate the well-known increase in tau phosphorylation (pTau) caused by the exposure to acute physical and psychological stressors. To test the hypothesis that reproductive status can impact hippocampal pTau induced by chronic stress, cohorts of virgin, lactating (4–5 days pp), and post-maternal (1-month post-weaned) rats were subjected to a daily 30-min episode of restraint stress for 14 days and were sacrificed either 20 min or 24 h after their last stress/handling episode. Western blot analysis of two well-characterized AD-relevant pTau epitopes (AT8 and PHF-1) and upstream pTau mechanisms (e.g. GSK3β) analysis, showed that stressed post-maternal rats have increased pTau in comparison to stressed lactating rats 20 min after their last stress episode. Furthermore, an increase in pTau was also seen 24 h after the last stress episode in stressed post-maternal rats in comparison to their non-stressed controls in the detergent-soluble fraction. GSK3 analysis showed an increase in total levels of GSK3β in virgin rats and an increase of inactive levels of GSK3β in post-maternal rats, which suggests a different stress response in pTau after the rat has gone through the maternal experience. Interestingly, post-maternal rats also presented the more variability in their estrous cycles in response to stress. Besides no differences in pTau, non-stressed lactating rats showed an increase in inactive GSK3β 24 h after the last handling episode. Immunohistochemical detection of the PHF-1 epitope revealed increased pTau in the CA4/hilar subfield of the hippocampus of virgin and post-maternal rats exposed to chronic stress shortly after their last stress episode. Overall, lactating rats remained unresponsive to chronic restraint stress. These results suggest increased sensitivity of the virgin and post-maternal rats to hippocampal stress-induced pTau with chronic restraint stress compared to lactating rats. Because no differences were detected in response to stress by lactating rats and an exaggerated response was observed in post-maternal rats, current results support the hypothesis that lactation affects tau processing in the brain of the female.

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pTau increases in the hippocampus of stressed virgin and especially post-maternal rats but not in that of lactating dams.

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The hippocampal area CA4 of virgin and post-maternal rats is most affected by the chronic restraint stress.

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GSK3β overall levels and activity are modified by the reproductive condition and stress.

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Reproductive experience modifies pTau induced by chronic stress.

A Systematic Review of the Association Between Psychological Stress and Dementia Risk in Humans

Background: It has been estimated that one third of dementia cases may be preventable through modifiable lifestyle interventions. Epidemiological evidence suggests a link between stressful life events and aging-related cognitive decline and dementia; however, inherent methodological limitations in examining subjective and biological measures of stress separately leads to interpretive constraints. Objective: The aim of the current study was to conduct a systematic review of the research literature investigating the effect of perceived and biological measures of stress on dementia risk. Methods: A systematic review was conducted of cohort, case-control, longitudinal prospective or retrospective studies examining the association between stress and risk of developing dementia. Studies were identified from a systematic search across major electronic databases from inception to February 2020. Results: Overall, 22 studies were identified including a total of 496,556 participants, approximately 50% were females, with sample sizes ranging from 62–270,977. There was considerable heterogeneity in the definition and measurement of stress. Most of the identified studies reported a significant positive association between stress and dementia risk. Conclusion: Evidenced from the current review is that personality traits linked to increased perceived stress and elevated reported perceived stress, are associated with greater statistical risk for dementia. However, this review highlights that caution must be exhibited in interpreting these findings, as methodological issues with confounding adjustment may mediate these results. Future research should focus on the investigation of stress on dementia risk with a full range of confounding adjustment, and on biological measures of stress.

Integrating Gene and Protein Expression Reveals Perturbed Functional Networks in Alzheimer's Disease

Asymptomatic and symptomatic Alzheimer’s disease (AD) subjects may present with equivalent neuropathological burdens but have significantly different antemortem cognitive decline rates. Using the transcriptome as a proxy for functional state, we selected 414 expression profiles of symptomatic AD subjects and age-matched non-demented controls from a community-based neuropathological study. By combining brain tissue-specific protein interactomes with gene networks, we identified functionally distinct composite clusters of genes that reveal extensive changes in expression levels in AD. Global expression for clusters broadly corresponding to synaptic transmission, metabolism, cell cycle, survival, and immune response were downregulated, while the upregulated cluster included largely uncharacterized processes. We propose that loss of EGR3 regulation mediates synaptic deficits by targeting the synaptic vesicle cycle. Our results highlight the utility of integrating protein interactions with gene perturbations to generate a comprehensive framework for characterizing alterations in the molecular network as applied to AD.

Canchi et al. reveal the transcriptomic dynamics of clinically and neuropathologically confirmed Alzheimer’s disease subjects by integrating brain tissue-specific proteome data with gene network analysis. They identify perturbed biological processes and provide insights into the interactions between molecular mechanisms in symptomatic Alzheimer’s disease.

Stress and Alzheimer's disease: A senescence link?

Chronic stress has been shown to promote numerous aging-related diseases, and to accelerate the aging process itself. Of particular interest is the impact of stress on Alzheimer's disease (AD), the most prevalent form of dementia. The vast majority of AD cases have no known genetic cause, making it vital to identify the environmental factors involved in the onset and progression of the disease. Age is the greatest risk factor for AD, and measures of biological aging such as shorter telomere length, significantly increase likelihood for developing AD. Stress is also considered a crucial contributor to AD, as indicated by a formidable body of research, although the mechanisms underlying this association remain unclear. Here we review human and animal literature on the impact of stress on AD and discuss the mechanisms implicated in the interaction. In particular we will focus on the burgeoning body of research demonstrating that senescent cells, which accumulate with age and actively drive a number of aging-related diseases, may be a key mechanism through which stress drives AD.

Does hearing loss lead to dementia? A review of the literature

Recent studies have revealed a correlation between aging-related hearing loss and the likelihood of developing Alzheimer Disease. However, it is not yet known if the correlation simply reflects the fact that these two disorders share common risk factors or whether there is a causal link between them. The answer to this question carries therapeutic implications. Unfortunately, it is not possible to study the question of causality between aging-related hearing loss and dementia in human subjects. Here, we evaluate the research surrounding induced-hearing loss in animal models on non-auditory cognition to help infer if there is any causal evidence linking hearing loss and a more general dementia. We find ample evidence that induction of hearing loss in animals produces cognitive decline, particularly hippocampal dysfunction. The data suggest that noise-exposure produces a toxic milieu in the hippocampus consisting of a spike in glucocorticoid levels, elevations of mediators of oxidative stress and excitotoxicity, which as a consequence induce cessation of neurogenesis, synaptic loss and tau hyperphosphorylation. These data suggest that hearing loss can lead to pathological hallmarks similar to those seen in Alzheimer's Disease and other dementias. However, the rodent data do not establish that hearing loss on its own can induce a progressive degenerative dementing illness. Therefore, we conclude that an additional "hit", such as aging, APOE genotype, microvascular disease or others, may be necessary to trigger an ongoing degenerative process such as Alzheimer Disease.

Neuroprotection of Tropical Fruit Juice Mixture via the Reduction of iNOS Expression and CRH Level in β-Amyloid-Induced Rats Model of Alzheimer's Disease

This study aimed to determine the effects of tropical fruit juice mixture (pomegranate, white guava, and Roselle) on biochemical, behavioral, and histopathological changes of β-amyloid- (Aβ-) induced rats. Formulation 8 (F8) of tropical fruit juice mixture was chosen for this present study due to its high phenolic content and antioxidant capacity. Forty Wistar male rats were divided into five groups: dPBS (sham-operated control), dAβ (Aβ control), JPBS (F8 and PBS), JAβ (F8 and Aβ), and IBFAβ (ibuprofen and Aβ). F8 (5 ml/kg BW), and ibuprofen (10 ml/kg BW) was given orally daily for four weeks before the intracerebroventricular infusion of Aβ for two weeks. Histological analysis and neuronal count of hippocampus tissue in the Cornu Ammonis (CA1) region showed that supplementation with F8 was able to prevent Aβ-induced tissue damage and neuronal shrinkage. However, no significant difference in locomotor activity and novel object recognition (NOR) percentage was detected among different groups at day 7 and day 14 following Aβ infusion. Only effect of time differences (main effect of day) was observed at day 7 as compared to day 14, where reduction in locomotor activity and NOR percentage was observed in all groups, with F (1, 7) = 6.940, p < 0.05 and F (1, 7) = 7.152, p < 0.05, respectively. Besides, the MDA level of the JAβ group was significantly lower (p < 0.01) than that of the dPBS group. However, no significant changes in SOD activity were detected among different groups. Significant reduction in plasma CRH level (p < 0.05) and iNOS expression (p < 0.01) in the brain was detected in the JAβ group as compared to the dAβ group. Hence, our current findings suggest that the tropical fruit juice mixture (F8) has the potential to protect the rats from Aβ-induced neurotoxicity in brain hippocampus tissue possibly via its antioxidant properties and the suppression of iNOS expression and CRH production.

Stress-Induced Neurodegeneration: The Potential for Coping as Neuroprotective Therapy

Stress responses are essential for survival, but become detrimental to health and cognition with chronic activation. Chronic hypothalamic-pituitary-adrenal axis release of glucocorticoids induces hypothalamic-pituitary-adrenal axis dysfunction and neuronal loss, decreases learning and memory, and modifies glucocorticoid receptor/mineralocorticoid receptor expression. Elderly who report increased stress are nearly 3 times more likely to develop Alzheimer's disease, have decreased global cognition and faster cognitive decline than those reporting no stress. Patients with mild cognitive impairment are more sensitive to stress compared to healthy elderly and those with Alzheimer's disease. Stress may also transduce neurodegeneration via the gut microbiome. Coping styles determine hippocampal mineralocorticoid receptor expression in mice, indicating that coping modifies cortisol's effect on the brain. Identifying neuroprotective coping strategies that lessen the burden of stress may prevent or slow cognitive decline. Treatments and education designed to reduce stress should be recognized as neuroprotective.

Corticotropin releasing factor-binding protein (CRF-BP) as a potential new therapeutic target in Alzheimer's disease and stress disorders

Alzheimer's disease is the most common cause of dementia and one of the most complex human neurodegenerative diseases. Numerous studies have demonstrated a critical role of the environment in the pathogenesis and pathophysiology of the disease, where daily life stress plays an important role. A lot of epigenetic studies have led to the conclusion that chronic stress and stress-related disorders play an important part in the onset of neurodegenerative disorders, and an enormous amount of research yielded valuable discoveries but has so far not led to the development of effective treatment strategies for Alzheimer's disease. Corticotropin-releasing factor (CRF) is one of the major hormones and at the same time a neuropeptide acting in stress response. Deregulation of protein levels of CRF is involved in the pathogenesis of Alzheimer's disease, but little is known about the precise roles of CRF and its binding protein, CRF-BP, in neurodegenerative diseases. In this review, we summarize the key evidence for and against the involvement of stress-associated modulation of the CRF system in the pathogenesis of Alzheimer's disease and discuss how recent findings could lead to new potential treatment possibilities in Alzheimer's disease by using CRF-BP as a therapeutic target.

GSK-3β and ERK1/2 incongruously act in tau hyperphosphorylation in SPS-induced PTSD rats

Post-traumatic stress disorder (PTSD) manifests in neurocognitive deficits in association with increased tau deposition, which mainly consist of phosphorylated tau in Alzheimer disease (AD) brain. However, the exact mechanism of PTSD inducing tau hyperphosphorylation remains unclear and therefore no effective treatment options are currently available. We here show that employing single prolonged stress (SPS), as a consensus PTSD model, induced a typical anxiety and abnormal hyperphosphorylation of tau at Ser202/Thr205 (AT8) and Ser404 but not at Ser199 and Ser396 in the hippocampus compared to the control rats. Furthermore, there was a decrease in the level of inactivated phosphorylated GSK-3β at Ser9, an increase in the level of activated phosphorylated GSK-3β at Thr216 and an obvious decrease in the level of activated phosphorylated ERK1/2, but no alterations in CaMKII and PP2A in hippocampus of SPS rats. On the other hand, the levels of both phosphorylated AKT and total SGK1, stress- and GSK-3β/ERK1/2-related proteins, were down-regulated. Interestingly, Overexpression of SGK1 increased the level of phosphorylated ERK1/2 and led to tau hyperphosphorylation at Ser199 and Ser396. These findings suggest that SPS exposure results in differential tau phosphorylation at different sites probably due to incongruous action between AKT-related GSK-3β activation and SGK1-related ERK1/2 inactivation, suggesting a link between SPS-induced PTSD and AD-associated tau pathogenic mechanisms.

Risk Factors for Alzheimer's Disease: Focus on Stress

In vulnerable individuals, chronic and persistent stress is an established risk factor for disorders that are comorbid with Alzheimer’s disease (AD), such as hypertension, obesity and metabolic syndrome, and psychiatric disorders. There are no disease-modifying drugs in the treatment of AD, and all phase-3 clinical trials with anti-amyloid drugs (e.g., β- or γ-secretase inhibitors and monoclonal antibodies) did not meet the primary endpoints. There are many reasons for the lack of efficacy of anti-amyloid drugs in AD, the most likely being a late start of treatment, considering that pathophysiological mechanisms underlying synaptic dysfunction and neuronal death begin several decades before the clinical onset of AD. The identification of risk factors is, therefore, an essential step for early treatment of AD with candidate disease-modifying drugs. Preclinical studies suggest that stress, and the resulting activation of the hypothalamic–pituitary–adrenal axis, can induce biochemical abnormalities reminiscent to those found in autoptic brain samples from individuals affected by AD (e.g., increases amyloid precursor protein and tau hyperphosphorylation). In this review, we will critically analyze the current knowledge supporting stress as a potential risk factor for AD.

Localization of amyloid beta peptides to locus coeruleus and medial prefrontal cortex in corticotropin releasing factor overexpressing male and female mice

A culmination of evidence from the literature points to the Locus Coeruleus (LC)-Norepinephrine system as an underappreciated and understudied area of research in the context of Alzheimer's Disease (AD). Stress is a risk factor for developing AD, and is supported by multiple clinical and preclinical studies demonstrating that amplification of the stress system disrupts cellular and molecular processes at the synapse, promoting the production and accumulation of the amyloid beta (Aβ42) peptide. Stress-induced activation of the LC is mediated by corticotropin releasing factor (CRF) and CRF receptors exhibit sex-biased stress signaling. Sex differences are evident in the neurochemical, morphological and molecular regulation of LC neurons by CRF, providing a compelling basis for the higher prevalence of stress-related disorders such as AD in females. In the present study, we examined the cellular substrates for interactions between Aβ and tyrosine hydroxylase a marker of noradrenergic somatodendritic processes in the LC, and Dopamine-β-Hydroxylase (DβH) in the infralimbic medial prefrontal cortex (ILmPFC) in mice conditionally overexpressing CRF in the forebrain (CRFOE) under a Doxycycline (DOX) regulated tetO promoter. CRFOE was sufficient to elicit a redistribution of Aβ peptides in the somatodendritic processes of the LC of male and female transgenic mice, without altering total Aβ42 protein expression levels. DOX treated groups exhibited lysosomal compartments with apparent lipofuscin and abnormal morphology, indicating potential dysfunction of these Aβ42-clearing compartments. In female DOX treated groups, swollen microvessels with lipid-laden vacuoles were also observed, a sign of blood-brain-barrier dysfunction. Finally, sex differences were observed in the prefrontal cortex, as females responded to DOX treatment with increased frequency of co-localization of Aβ42 and DβH in noradrenergic axon terminals compared to vehicle treated controls, while male groups showed no significant changes. We hypothesize that the observed sex differences in Aβ42 distribution in this model of CRF hypersignaling is based on increased responsivity of female rodent CRFR1 in the LC. Aβ42 production is enhanced during increased neuronal activation, therefore, the excitation of DOX treated female CRFOE LC neurons projecting to the mPFC may exhibit more frequent co-localization with Aβ due to increased neuronal activity of noradrenergic neurons.

Exposure to a single immobilization or lipopolysaccharide challenge increases expression of genes implicated in the development of Alzheimer's disease in the mice brain cortex

OBJECTIVES

Despite extensive research efforts, mechanisms participating on development of Alzheimer's disease (AD) are covered only partially. Data from the last decades indicate that various stressors, as etiological factors, may play a role of in the AD. Therefore, we investigated the effect of two acute stressors, immobilization (IMO) and lipopolysaccharide (LPS), on the AD-related neuropathology.

METHODS

Adult C57BL/6J mice males were exposed to a single IMO stress or a single intraperitoneal injection of LPS (250 µg/kg body weight). After terminating the experiments, the brains were removed and their cortices isolated. Gene expression of pro-inflammatory cytokines, as well as expression of genes implicated in the AD neuropathology were determined. In addition, mediators related to the activation of the microglia, monocytes, and perivascular macrophages were determined in brain cortices, as well.

RESULTS

In comparison with the control animals, we found increased gene expression of proinflammatory mediators in mice brain cortex in both IMO and LPS groups. In stressed animals, we also showed an increased expression of genes related to the AD neuropathology, as well as positive correlations between genes implicated in AD development and associated neuroinflammation.

CONCLUSIONS

Our data indicate that acute exposure to a strong IMO stressor, composed of the combined physical and psychological challenges, induces similar inflammatory and other ADrelated neuropathological changes as the immune LPS treatment. Our data also indicate that cytokines are most likely released from the peripheral immune cells, as we detected myeloid cells activity, without any microglia response. We hypothesize that stress induces innate immune response in the brain that consequently potentiate the expression of genes implicated in the AD-related neuropathology.

Memantine Differentially Regulates Tau Phosphorylation Induced by Chronic Restraint Stress of Varying Duration in Mice

Exposure to chronic psychiatric stress has been linked to Alzheimer's disease-related tau hyperphosphorylation and abnormalities in glutamate neurotransmission. However, the pathological relationship between glutamatergic dysfunction and tau phosphorylation in the cerebral cortex under chronic psychiatric stress is not fully understood. The present study investigated the effects of memantine (MEM, 5 and 10 mg/kg), an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, on chronic restraint stress- (CRS-) induced tau phosphorylation in mice. CRS administered for 16 or 28 consecutive days (1 h daily) induced significant tau phosphorylation in the brain. MEM treatment suppressed the elevation of phosphorylated tau (P-tau) levels induced by 16-day CRS in a dose-dependent manner. P-tau reduction was accompanied by the attenuation of the upregulation of GSK3β and CDK5 expression and the downregulation of PP2A activity induced by CRS. Additionally, MEM reduced CRS-induced upregulation of NMDA receptor subunit levels (GluN2A, GluN2B) in the frontal cortex. However, MEM markedly enhanced tau phosphorylation in the frontal cortex and other cerebral cortical regions following 28 days of CRS. The stimulatory effect of MEM on CRS-induced tau phosphorylation was correlated with increased activities of AKT, JNK, and GSK3β, inactivation of PP2A, and downregulation of Pin1 and HSP70. Moreover, MEM did not effectively reverse the NMDA receptor upregulation induced by 28-day CRS and even increased GluN2B subunit levels. In contrast to the duration-dependent effects of MEM on P-tau levels, MEM produced an anxiolytic effect in both regimens as revealed by elevated plus maze testing. However, MEM did not affect the body weight reduction induced by CRS. Thus, MEM exerts distinctive effects on CRS-induced tau phosphorylation, which might be related to the expression of GluN2B. The differential effects of MEM on P-tau levels have crucial implications for its clinical application.

Stress and the Etiopathogenesis of Alzheimer's Disease and Depression

Alzheimer's disease (AD) is a multifactorial neurodegenerative disorder with a complex physiopathology whose initiators are poorly defined. Accumulating clinical and experimental evidence suggests a causal role of lifetime stress in AD. This chapter summarizes current knowledge about how chronic stress and its accompanying high levels of glucocorticoid (GC) secretion, trigger the two main pathomechanisms of AD: (i) misprocessing of amyloid precursor protein (APP) and the generation of amyloid beta (Aβ) and (ii) Tau hyperphosphorylation and aggregation. Given that depression is a well-known stress-related illness, and the evidence that depression may precede AD, this chapter also explores neurobiological mechanisms that may be common to depressive and AD pathologies. This review also discusses emerging insights into the role of Tau and its malfunction in disrupting neuronal cascades and neuroplasticity and, thus triggering brain pathology.

Tauopathy and neurodegeneration: A role for stress

Neurodegenerative diseases are characterized by an irreversible and progressive loss of neuronal structure and function. While many alterations to normal cellular processes occur during neurodegeneration, a pathological accumulation of aggregated proteins constitutes a hallmark of several neurodegenerative disorders. Alzheimer's disease, specifically, is pathologically defined by the formation of amyloid plaques and tangles of hyperphosphorylated tau protein. Stress has emerged as an important factor in the development and progression of neurodegenerative diseases, including Alzheimer's. Very little is known, however, regarding the effects of stress on the mechanisms controlling abnormal protein aggregation and clearance. Chronic stress activates the hypothalamic-pituitary-adrenal (HPA) axis, causing an excessive secretion of glucocorticoids that are capable of impacting diverse physiological and cellular processes. The present review focuses on the influence of stress on a key feature of Alzheimer's disease pathology, emphasizing the relationship between tau phosphorylation and accumulation and its connection to HPA axis dysfunction.

Central Role of Glucocorticoid Receptors in Alzheimer's Disease and Depression

Alzheimer’s disease (AD) is the principal neurodegenerative pathology in the world displaying negative impacts on both the health and social ability of patients and inducing considerable economic costs. In the case of sporadic forms of AD (more than 95% of patients), even if mechanisms are unknown, some risk factors were identified. The principal risk is aging, but there is growing evidence that lifetime events like chronic stress or stress-related disorders may increase the probability to develop AD. This mini-review reinforces the rationale to consider major depressive disorder (MDD) as an important risk factor to develop AD and points the central role played by the hypothalamic-pituitary-adrenal (HPA) axis, glucocorticoids (GC) and their receptors (GR) in the etiology of MDD and AD. Several strategies directly targeting GR were tested to neutralize the HPA axis dysregulation and GC overproduction. Given the ubiquitous expression of GR, antagonists have many undesired side effects, limiting their therapeutic potential. However, a new class of molecules was developed, highly selective and acting as modulators. They present the advantage to selectively abrogate pathogenic GR-dependent processes, while retaining beneficial aspects of GR signaling. In fact, these “selective GR modulators” induce a receptor conformation that allows activation of only a subset of downstream signaling pathways, explaining their capacity to combine agonistic and antagonistic properties. Thus, targeting GR with selective modulators, alone or in association with current strategies, becomes particularly attractive and relevant to develop novel preventive and/or therapeutic strategies to tackle disorders associated with a dysregulation of the HPA axis.

The Corticotropin-Releasing Factor Family: Physiology of the Stress Response

The physiological stress response is responsible for the maintenance of homeostasis in the presence of real or perceived challenges. In this function, the brain activates adaptive responses that involve numerous neural circuits and effector molecules to adapt to the current and future demands. A maladaptive stress response has been linked to the etiology of a variety of disorders, such as anxiety and mood disorders, eating disorders, and the metabolic syndrome. The neuropeptide corticotropin-releasing factor (CRF) and its relatives, the urocortins 1–3, in concert with their receptors (CRFR1, CRFR2), have emerged as central components of the physiological stress response. This central peptidergic system impinges on a broad spectrum of physiological processes that are the basis for successful adaptation and concomitantly integrate autonomic, neuroendocrine, and behavioral stress responses. This review focuses on the physiology of CRF-related peptides and their cognate receptors with the aim of providing a comprehensive up-to-date overview of the field. We describe the major molecular features covering aspects of gene expression and regulation, structural properties, and molecular interactions, as well as mechanisms of signal transduction and their surveillance. In addition, we discuss the large body of published experimental studies focusing on state-of-the-art genetic approaches with high temporal and spatial precision, which collectively aimed to dissect the contribution of CRF-related ligands and receptors to different levels of the stress response. We discuss the controversies in the field and unravel knowledge gaps that might pave the way for future research directions and open up novel opportunities for therapeutic intervention.

Stress induced neural reorganization: A conceptual framework linking depression and Alzheimer's disease

Chronic stress is a risk factor for a number of physiological disorders including cardiovascular disease, obesity and gastrointestinal disorders, as well as psychiatric and neurodegenerative disorders. There are a number of underlying molecular and cellular mechanisms altered in the course of chronic stress, which may increase the vulnerability of individuals to develop psychiatric disorders such as depression, and neurodegenerative disorders such as Alzheimer's Disease (AD). This is evident in the influence of stress on large-scale brain networks, including the resting state Default Mode Network (DMN), the effects of stress on neuronal circuitry and architecture, and the cellular and molecular adaptations to stress, which may render individuals with stress related psychiatric disorders more vulnerable to neurodegenerative disease later in life. These alterations include decreased negative feedback inhibition of the hypothalamic pituitary axis (HPA) axis, decreased dendritic arborization and spine density in the prefrontal cortex (PFC) and hippocampus, and the release of proinflammatory cytokines, which may suppress neurogenesis and promote neuronal cell death. Each of these factors are thought to play a role in stress-related psychiatric disease as well as AD, and have been observed in clinical and post-mortem studies of individuals with depression and AD. The goal of the current review is to summarize clinical and preclinical evidence supporting a role for chronic stress as a putative link between neuropsychiatric and neurodegenerative disease. Moreover, we provide a rationale for the importance of taking a medical history of stress-related psychiatric diseases into consideration during clinical trial design, as they may play an important role in the etiology of AD in stratified patient populations.

Beta estradiol and norepinephrine treatment of differentiated SH-SY5Y cells enhances tau phosphorylation at (Ser396) and (Ser262) via AMPK but not mTOR signaling pathway

Hyperphosphorylation of tau is one of the main hallmarks for Alzheimer's disease (AD) and many other tauopathies. Norepinephrine (NE), a stress-related hormone and 17-β-estradiol (E2) thought to influence tau phosphorylation (p-tau) and AD pathology. The controversy around the impact of NE and E2 requires further clarification. Moreover, the combination effect of physiological and psychological stress and estrogen alteration during menopause, which affect p-tau, has not been addressed. Exposure to E2 is believed to reduce NE release, however, the link between these two hormones and AD at cellular level was also remained unknown. Here, we examined whether NE and E2 treatment of differentiated SH-SY5Y cells affected tau phosphorylation. The involvement of adenosine monophosphate kinase protein kinase (AMPK) and target of Rapamycin (mTOR) as the possible mechanisms, underlying this effect was also investigated. Subsequent to SH-SY5Y differentiation to mature neurons, we treated the cells with NE, E2 and NE plus E2 in presence and absence of Compound C and Rapamycin. Cell viability was not affected by our treatment while our Western blot and immunofluorescent findings showed that exposure to NE and E2 separately, and in combination enhanced p-tau (Ser³⁹⁶) and (Ser²⁶²)/tau but not (Ser²⁰²/Thr²⁰⁵)/tau. Blocking AMPK by Compound C reduced p-tau (Ser³⁹⁶) and (Ser²⁶²), while GSK-3β and PP2A activities were remained unchanged. We also found that blocking mTOR by Rapamycin did not change increased p-tau (Ser³⁹⁶) and (Ser²⁶²) due to NE + E2 treatment. Collectively, our results suggested that tau hyperphosphorylation due to exposure to NE/E2 was mediated by AMPK, the main energy regulator of cells during stress with no significant involvement of mTOR, GSK-3β and PP2A.

A preclinical perspective on the enhanced vulnerability to Alzheimer's disease after early-life stress

Stress experienced early in life (ES), in the form of childhood maltreatment, maternal neglect or trauma, enhances the risk for cognitive decline in later life. Several epidemiological studies have now shown that environmental and adult life style factors influence AD incidence or age-of-onset and early-life environmental conditions have attracted attention in this respect. There is now emerging interest in understanding whether ES impacts the risk to develop age-related neurodegenerative disorders, and their severity, such as in Alzheimer's disease (AD), which is characterized by cognitive decline and extensive (hippocampal) neuropathology. While this might be relevant for the identification of individuals at risk and preventive strategies, this topic and its possible underlying mechanisms have been poorly studied to date. In this review, we discuss the role of ES in modulating AD risk and progression, primarily from a preclinical perspective. We focus on the possible involvement of stress-related, neuro-inflammatory and metabolic factors in mediating ES-induced effects on later neuropathology and the associated impairments in neuroplasticity. The available studies suggest that the age of onset and progression of AD-related neuropathology and cognitive decline can be affected by ES, and may aggravate the progression of AD neuropathology. These relevant changes in AD pathology after ES exposure in animal models call for future clinical studies to elucidate whether stress exposure during the early-life period in humans modulates later vulnerability for AD.

Sex differences in chronic stress responses and Alzheimer's disease

Clinical studies indicate that Alzheimer's disease (AD) disproportionately affects women in both disease prevalence and severity, but the mechanisms underlying this sex divergence are unknown. Though some have suggested this difference in risk is a reflection of known differences in longevity between men and women, mounting clinical and preclinical evidence supports women also having intrinsic susceptibilities towards the disease. While a number of potential risk factors have been hypothesized to affect these differences in risks, none have been definitively verified. In this review, we discuss a novel hypothesis whereby women's susceptibility to chronic stress also mediates increased risk for AD. As stress is a risk factor for AD, and women are twice as likely to develop mood disorders where stress is a major etiology, it is possible that sex dimorphisms in stress responses contribute to the increase in women with AD. In line with this, sex divergence in biochemical responses to stress have been noted along the hypothalamic-pituitary-adrenal (HPA) axis and among known molecular effectors of AD, with crosstalk between these processes also being likely. In addition, activation of the cortical corticotrophin-releasing factor receptor 1 (CRF1) signaling pathway leads to distinct female-biased increases in molecules associated with AD pathogenesis. Therefore, the different biochemical responses to stress between women and men may represent an intrinsic, sex-dependent risk factor for AD.

The relationship between stress and Alzheimer's disease

Stress is critically involved in the development and progression of disease. From the stress of undergoing treatments to facing your own mortality, the physiological processes that stress drives have a serious detrimental effect on the ability to heal, cope and maintain a positive quality of life. This is becoming increasingly clear in the case of neurodegenerative diseases. Neurodegenerative diseases involve the devastating loss of cognitive and motor function which is stressful in itself, but can also disrupt neural circuits that mediate stress responses. Disrupting these circuits produces aberrant emotional and aggressive behavior that causes long-term care to be especially difficult. In addition, added stress drives progression of the disease and can exacerbate symptoms. In this review, I describe how neural and endocrine pathways activated by stress interact with ongoing neurodegenerative disease from both a clinical and experimental perspective.

ATP-sensitive potassium-channel inhibitor glibenclamide attenuates HPA axis hyperactivity, depression- and anxiety-related symptoms in a rat model of Alzheimer's disease

Affective disorders including depression and anxiety are among the most prevalent behavioral abnormalities in patients with Alzheimer's disease (AD), which affect the quality of life and progression of the disease. Dysregulation of the hypothalamic-pituitary-adrenal-(HPA) axis has been reported in affective disorders and AD. Recent studies revealed that current antidepressant drugs are not completely effective for treating anxiety- and depression-related disorders in people with dementia. ATP-sensitive-potassium-(K_ATP) channels are well-known to be involved in AD pathophysiology, HPA axis function and the pathogenesis of depression and anxiety-related behaviors. Thus, targeting of K_ATP channel may be a potential therapeutic strategy in AD. Hence, we investigated the effects of intracerebroventricular injection of Aβ25-35 alone or in combination with glibenclamide, K_ATP channel inhibitor on depression- and anxiety-related behaviors as well as HPA axis response to stress in rats. To do this, non-Aβ25-35- and Aβ25-35-treated rats were orally treated with glibenclamide, then the behavioral consequences were assessed using sucrose preference, forced swim, light-dark box and plus maze tests. Stress-induced corticosterone levels following forced swim and plus maze tests were also evaluated as indicative of abnormal HPA-axis-function. Aβ25-35 induced HPA axis hyperreactivity and increased depression- and anxiety-related symptoms in rats. Our results showed that blockade of K_ATP channels with glibenclamide decreased depression- and anxiety-related behaviors by normalizing HPA axis activity in Aβ25-35-treated rats. This study provides additional evidence that Aβ administration can induce depression- and anxiety-like symptoms in rodents, and suggests that K_ATP channel inhibitors may be a plausible therapeutic strategy for treating affective disorders in AD patients.

Tau Phosphorylation in Female Neurodegeneration: Role of Estrogens, Progesterone, and Prolactin

Sex differences are important to consider when studying different psychiatric, neurodevelopmental, and neurodegenerative disorders, including Alzheimer's disease (AD). These disorders can be affected by dimorphic changes in the central nervous system and be influenced by sex-specific hormones and neuroactive steroids. In fact, AD is more prevalent in women than in men. One of the main characteristics of AD is the formation of neurofibrillary tangles, composed of the phosphoprotein Tau, and neuronal loss in specific brain regions. The scope of this work is to review the existing evidence on how a set of hormones (estrogen, progesterone, and prolactin) affect tau phosphorylation in the brain of females under both physiological and pathological conditions.

Anxiety-like behavior as an early endophenotype in the TgF344-AD rat model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by progressive cognitive decline and the presence of aggregates of amyloid beta (plaques) and hyperphosphorylated tau (tangles). Early diagnosis through neuropsychological testing is difficult due to comorbidity of symptoms between AD and other types of dementia. As a result, there is a need to identify the range of behavioral phenotypes expressed in AD. In the present study, we utilized a transgenic rat (TgF344-AD) model that bears the mutated amyloid precursor protein as well as presenilin-1 genes, resulting in progressive plaque and tangle pathogenesis throughout the cortex. We tested young adult male and female TgF344-AD rats in a spatial memory task in the Morris water maze and for anxiety-like behavior in the elevated plus-maze. Results indicated that regardless of sex, TgF344-AD rats exhibited increased anxiety-like behavior in the elevated plus-maze, which occurred without significant deficits in the spatial memory. Together, these results indicate that enhanced anxiety-like behavior represents an early-stage behavioral marker in the TgF344-AD rat model.

Effects of chronic noise on the corticotropin-releasing factor system in the rat hippocampus: relevance to Alzheimer's disease-like tau hyperphosphorylation

Background

Chronic noise exposure has been associated with tau hyperphosphorylation and Alzheimer’s disease (AD)-like pathological changes, but the underlying mechanism is unknown. In this study, we explored the effects of long-term noise exposure on the corticotropin-releasing factor (CRF) system in the hippocampus and its role in noise-induced tau phosphorylation.

Methods

Sixty-four rats were randomly divided into the noise-exposed group and the control group, and rats in the exposure group were exposed to 95 dB SPL white noise for 30 consecutive days. The levels of CRF, CRFR1, CRFR2, and total tau and phosphorylated tau (p-tau) at Ser396 (S396) and Thr205 (T205) in the hippocampus were measured at different time points after the final noise exposure. The co-localized distribution of CRF and p-tau (T205) in the hippocampus was evaluated using double-labeling immunofluorescence.

Results

Long-term exposure to noise for 30 consecutive days significantly increased the expression of CRF and CRFR1 and their mRNAs levels in the hippocampus, which persisted for 7 days after final exposure. In contrast, CRFR2 was raised for 3–7 days following the last exposure. These alterations were also concomitant with the phosphorylation of tau at S396 and T205. Furthermore, there was co-localization of p-tau and CRF in hippocampal neurons.

Conclusion

Chronic noise leads to long-lasting increases in the hippocampal CRF system and the hyperphosphorylation of tau in the hippocampus. Our results also provide evidence for the involvement of the CRF system in noise-induced AD-like neurodegeneration.

Atypical, non-standard functions of the microtubule associated Tau protein

Since the discovery of the microtubule-associated protein Tau (MAPT) over 40 years ago, most studies have focused on Tau's role in microtubule stability and regulation, as well as on the neuropathological consequences of Tau hyperphosphorylation and aggregation in Alzheimer's disease (AD) brains. In recent years, however, research efforts identified new interaction partners and different sub-cellular localizations for Tau suggesting additional roles beyond its standard function as microtubule regulating protein. Moreover, despite the increasing research focus on AD over the last decades, Tau was only recently considered as a promising therapeutic target for the treatment and prevention of AD as well as for neurological pathologies beyond AD e.g. epilepsy, excitotoxicity, and environmental stress. This review will focus on atypical, non-standard roles of Tau on neuronal function and dysfunction in AD and other neurological pathologies providing novel insights about neuroplastic and neuropathological implications of Tau in both the central and the peripheral nervous system.